The present invention relates to a heterojunction bipolar transistor (hereinafter, abbreviated as HBT) and manufacturing method for HTB as well as a communication device with HTB.
Conventionally, in HBTs, there has been adopted a barrier structure in which part of an emitter layer is left on an external base region, which is defined as a region where a base layer outwardly extends from an emitter region, so that holes within the base layer are inhibited from reaching a surface of the external base on the external base region. By doing so, it becomes possible to prevent decrease of reliability due to recombination of electrons diffused from the emitter layer with holes in the base layer, which combination occurs on the surface of the external base region.
The emitter layer left on the external base region needs to be of such a thickness that the emitter layer is depleted within a bias voltage range normal for transistors in order that current does not leak between an emitter and a base through the left emitter layer. Therefore, for more reliable formation of this thickness, it has been conventional practice to form a layer using V-group phosphorus as the emitter layer, thereby providing an epitaxial structure that enables selective etching.
As an example of conventional HBTs, an HBT structure of Japanese Patent Laid-Open Publication HEI 6-244195 is shown in FIG. 11. Referring to FIG. 11, reference numeral 1 denotes a semi-insulating GaAs substrate, 2 denotes a GaAs collector contact layer, 3 denotes a GaAs collector layer, 4 denotes a GaAs base layer, 5 denotes an InGaP emitter layer, 6 denotes an AlGaAs emitter layer, 7 denotes an AlGaAs graded layer, 8 denotes a GaAs cap layer, 9 denotes an InGaAs contact layer, 10 denotes an emitter electrode, 11 denotes a base electrode, and 12 denotes a collector electrode 12. In this case, the emitter layer 5 left on the external base region in the GaAs base layer 4 is very thin, 20 nm.
However, the conventional HBT disclosed in Japanese Patent Laid-Open Publication HEI 6-244195 has the following disadvantages.
The HBT having the structure shown in FIG. 11 is fabricated as follows. First, on the semi-insulating GaAs substrate 1, individual layers from the GaAs collector contact layer 2 to the InGaAs contact layer 9 are stacked sequentially. Next, the InGaAs contact layer 9, the GaAs cap layer 8, the AlGaAs graded layer 7 and the AlGaAs emitter layer 6 are etched by an etchant of phosphorus acid/hydrogen peroxide mixed aqueous solution, and the etching is ended selectively at the InGaP emitter layer 5. Subsequently, a resist pattern 13 is formed so as to cover the external base region, and the InGaP emitter layer 5 is etched by an etchant of hydrochloric acid/phosphorus acid mixed etchant, the etching being ended the GaAs base layer 4. Then, the GaAs base layer 4 and the GaAs collector layer 3 are etched again by an etchant of phosphorus acid/hydrogen peroxide mixed aqueous solution.
However, the HBT formed in this way, actually, has the InGaP emitter layer 5 overhanging at its ends in a protruded form by a width A, as shown in FIG. 12. For the reason that in the process of etching the GaAs base layer 4 and the GaAs collector layer 3 with the etchant of phosphorus acid/hydrogen peroxide mixed aqueous solution, the GaAs base layer 4 and the GaAs collector layer 3 are etched even at their side faces, while the InGaP emitter layer 5 is not etched
In that case, the InGaP emitter layer 5, which is set to such a thickness that the layer is depleted in the external base region, has quite a small thickness as thin as about 20 nm. Accordingly, in the etching process of the GaAs base layer 4 and the GaAs collector layer 3 or in later processes, the protrusion 14 may be damaged, allowing moisture or the like to penetrate through the broken places, which would cause decrease in yield and deterioration of reliability.
Therefore, in order to avoid these disadvantages, it is possible to remove the protrusion 14 of the InGaP emitter layer 5 by etching with a hydrochloric acid/phosphorus acid mixed etchant. This etching is conducted after the GaAs base layer 4 and the GaAs collector layer 3 are etched with the etchant of phosphorus acid/hydrogen peroxide mixed aqueous solution and before the resist pattern 13 is removed. In this case, however, side etching of the protrusion 14 occurs during the etching as shown in FIG. 13, so that the end of the InGaP emitter layer 5 are located inside the end of the GaAs base layer 4. As a result, the base layer 4 is exposed in an outer portion of the external base region. For this reason, the base electrode ohmic material directly contacts the base layer 4 outside the external base region. Consequently, when the base electrode ohmic material is diffused via the emitter layer 5 into the base layer 4 with ohmic junction, the base electrode ohmic material is simultaneously diffused via the base layer 4 up to the collector layer 3 in the outer portion of the external base region where the base layer 4 is exposed, as shown by broken line in FIG. 13. Accordingly, there arises a failure that a leak current flows between base and collector.
In particular, when the external base region is reduced in area for enhancement of the HBT performance, it becomes even more difficult to form the base electrode 11 while avoiding the upper surface of the base layer 4 from being exposed to the base electrode ohmic material due to the side etching of the emitter layer 5. As a result, the base electrode 11 comes into contact with the exposed portion of the base layer 4, giving rise to the failure that a leak current flows between base and collector.
As described above, conventional HBTs are low in reliability whether the protrusion is present in HBT or the protrusion has been removed by etching. Accordingly, the conventional HBTs have disadvantages of frequent failures and low durability when the conventional HBTs are used for power amplifiers in portable communication devices, which are used in wide-range environments and particularly which cannot sufficiently afford to provide for air-tightness because of demands for lightening of weight and reduction of size.
It is therefore an object of the present invention to provide an HBT as well as a manufacturing method therefor which can prevent damage of a protrusion of an emitter layer present on an external base region of the HBT, and to provide a communication device using the HBT.
In order to achieve the above object, in a first aspect of the invention, there is provided a heterojunction bipolar transistor in which an emitter mesa portion is formed of emitter layers in a mesa shape on a base layer, the emitter mesa portion having a lower layer of the emitter layers to extend up to an upper surface of an external base region which is a region of the base layer outside the base layer under the emitter mesa portion, comprising: an emitter layer formed of the lower layer in a first region present on the external base region located and next to the emitter mesa portion; and emitter layers including the lower emitter layer and forming a second region located outside the first region, wherein the emitter layers in the second region is thicker than emitter layer in the first region.
Normally, thickness of the lower layer of the emitter mesa portion extending to the external base region is set to such a thickness that an emitter layer of the lower layer is depleted within a normal bias voltage range to prevent current leaks between an emitter and a base through the emitter layer. For this purpose, it is necessary to deplete the emitter layer within a range from a base electrode formed on the external base region up to the emitter mesa portion. The rest of the emitter layer present on the external base region may be made thick without causing any problem.
With this constitution, as to thickness of the emitter layer or layers on the external base region, the emitter layers in the second region located outside the first region is thicker than the emitter layer in the first region located next to the emitter mesa portion. Therefore, the emitter layer in the first region can be set to such a thickness that the depletion can be achieved, while the emitter layers in the second region can be set to such a thickness that the strength of the protrusion formed by the emitter layers overhanging outside from the external base region is increased.
In one embodiment, there is a protrusion in the second region, the protrusion overhanging outside the external base region.
With this constitution, there is the protrusion, which overhangs outside the external base region, in the second region. Therefore, there is no part where the base layer is exposed in the external base region, so that distance between the base layer and a base ohmic electrode to be formed later in the external base region becomes generally uniform. Thus, even when mis-alignment has occurred during formation of the base ohmic electrode on the external base region, the base electrode material is uniformly diffused into the base layer in the depthwise direction thereof. As a result, it can be prevented that the base ohmic electrode comes into direct contact with the base layer to cause the base electrode material to diffuse up to the collector layer. Thus, occurrence of leak currents between base and collector can be prevented.
In one embodiment, a boundary between the first region and the second region is present on the external base region.
With this constitution, since the boundary between the first region and the second region is present on the external base region, the emitter layers in the second region has been overlaid on the external base region. Therefore, strength of the protrusion outgoing from the external base region is increased. In particular, the strength of the protrusion is improved when the emitter layers in the second region are overlaid over the entire periphery of the external base region or the longitudinal peripheries of the external base region.
In one embodiment, a base electrode is provided on the second region ranged from the periphery of the first region on the external base region to the protrusion.
Reduction in an area of the external base region for characteristic improvement makes it difficult to form a base electrode or to attain connections between the base electrode and base leads. In this embodiment, however, the base electrode is provided on the second region ranged from the periphery of the first region on the external base region to the protrusion. Therefore, the base electrode can be formed with a large pattern. Therefore, even when characteristic improvement is intended by reducing the area of the external base region, the base electrode can be formed stably. Moreover, an alignment margin can be taken in formation of the base lead. Thus, manufacturing yield is further improved. Moreover, the protrusion can be reinforced more strongly by the base electrode formed on the protrusion.
In the HBT of the first aspect of the invention, a surface layer of the first region and a surface layer of the second region are made of different materials, respectively.
With this constitution, since the surface layer of the first region and the surface layer of the second region are formed from different materials, thickness of the first region can be easily set by performing a selective etching which selectively etches to the different materials.
In one embodiment, the emitter layer in the first region has a thickness of not less than 5 nm and not more than 25 nm, and the emitter layers in the second region have a total thickness of not less than 40 nm.
With this constitution, since the emitter layer in the first region is set to a thickness not less than 5 nm and not more than 25 nm, the emitter layer in the vicinity of the emitter mesa portion is depleted within the normal bias voltage range. Further, since the emitter layers in the second region are totally set to a thickness of not less than 40 nm, the emitter layers in the second region, in particular, the protrusion is reinforced enough.
According to a second aspect of the present invention, there is provided a method for manufacturing a heterojunction bipolar transistor, comprising the steps of:
forming emitter layers on a base layer; forming an emitter mesa portion by forming the emitter layers into a mesa shape with lower layers of the emitter layers left at a first specified thickness; forming a mask over a region in an external base region, which is a region of the base layer outside the base layer under the emitter mesa portion, and distant from a side wall of the emitter mesa portion by a specified distance; and etching the lower layers on the external base region to make an emitter layer having a second specified thickness with the mask.
With this constitution, the second specified thickness can be set to such a thickness that the depletion can be achieved within the normal bias voltage range, while the first specified thickness can be set to such a thickness that the protrusion formed by the lower emitter layers on the external base region overhanging outside is increased in strength. Thus, an HBT superior in characteristics and high in yield can be formed easily by film deposition technique and photoetching technique.
In one embodiment, the mask is a base electrode.
With this constitution, since the base electrode is used as the mask, the region by a specified distance apart from the side wall of the emitter mesa portion can be formed in self alignment with the base electrode. Thus, the region for the emitter layer of the second specified thickness can be formed with excellent repeatability.
In one embodiment, the mask is formed of an insulating film.
With this constitution, an insulating film is used as the mask. Thus, the region of the second specified thickness can be formed with excellent repeatability by film deposition technique and etching technique.
In one embodiment, the step of forming the mask includes the steps of: forming an insulating film on the external base region and the side wall of the emitter mesa portion; and removing by etching the insulating film present on the side wall of the emitter mesa portion and the insulating film present within the specified distance from the side wall of the emitter mesa portion.
With this constitution, an insulating film is formed at a step gap of the emitter mesa portion formed in the external base region, and this insulating film is etched. In this connection, during the formation of the insulating film, because the step gap portion differs in composition, composition ratio or density from the other portions, the insulating film is etched at a higher etching rate in the step gap portion than in the other portions, so that only the insulating film present on the side wall of the emitter mesa portion and the insulating film present in the vicinity of the side wall of the emitter mesa portion are removed by etching. Therefore, by using the mask formed in this way, the region of the second specified thickness is formed in self alignment around the emitter mesa portion and thus formed with excellent repeatability.
In a third aspect for the present invention, there is provided a communication device using the HBT as described above.
The HBT in this aspect of the invention is free from damage of the protrusion formed by the emitter layers overhanging outside the external base region, as well as free from penetration of moisture through damaged places. With this constitution, HBTs having high characteristics as described above are used for power amplifiers of communication devices. Therefore, those communication devices to be used under various environments are improved in durability. In particular, for portable communication devices for which lower weight and smaller size are demanded, noticeably great effects can be obtained because of their insufficiency in air-tightness measures.